1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine or printer of an electrophotographic system, and, more particularly, to a sheet conveying apparatus and an image forming apparatus having this sheet conveying apparatus of a suctioning-conveying system using endless conveying belts.
2. Description of the Related Art
Recently, image forming apparatuses such as copying machines, printers and facsimile machines using an electrophotographic system are spreading. Further, these image forming apparatuses are required to output all sheets such as thin paper, thick paper and coated paper while maintaining high productivity (a number of sheets to form images per unit time), and, moreover, provide durability for maintaining quality in a long term of use.
Incidentally, in a sheet conveying portion, paper dust (sheet dust) is produced from sheets when rollers or guides which convey the sheets contact sheets. Further, with the electrophotographic system image forming apparatuses, part of toner used to form images remains in a photoreceptor (this is referred to as “transfer remaining toner”), and this transfer remaining toner flies before it is cleaned. Furthermore, there are cases where such paper dust and transfer remaining toner accumulate in sheet conveying portions which convey sheets to which toner is transferred, and the paper dust and transfer remaining toner additionally accumulated adhere to sheets to be conveyed and are heated and pressured by a fixing apparatus, thereby causing a problem that quality of images becomes poor. In addition, if paper dust and transfer remaining toner which adhere to sheets are little, image quality does not become poor even when the sheets are heated and pressured by the fixing apparatus. However, image quality becomes poor when a great amount of accumulated paper dust and transfer remaining toner collectively adhere to sheets at one time.
To solve this problem, a sheet conveying apparatus is proposed which is configured to make a cleaning member, which cleans sheet conveying surfaces of endless belts, slidably contact belt surfaces of the conveying belts. This technique is disclosed in Japanese Patent Laid-open No. 2006-131375.
With a conventional sheet conveying apparatus which cleans conveying belts using a cleaning member, the cleaning member slidably contacts the belt surfaces of the conveying belts at all times. Therefore, the belt surfaces of the conveying belts gradually become rough due to friction with the cleaning member in a long period of use and significant unevenness is formed. A great amount of paper dust and transfer remaining toner accumulate in the uneven portions, and therefore the cleaning member cannot sufficiently clean the sheet conveying surfaces. Particularly, with a configuration of a suctioning-conveying system of attracting a sheet to sheet conveying surfaces by applying negative pressure of air to conveying belts and conveying the sheet, a plurality of inlet holes for suctioning air is formed in the conveying belts, and therefore corner portions of the inlet holes are scraped and a great amount of paper dust and transfer remaining toner accumulate in these scraped portions. Further, contact pressure between the conveying belts and a sheet increases because the sheet is suctioned by means of negative pressure, and a great amount of paper dust and transfer remaining toner is likely to adhere to sheets collectively. Hence, the above problem that image quality becomes poor cannot be solved reliably even if a cleaning member is used.